Cemented carbide for applications such as seal rings, bearings, bushings, hot rolls, etc., should have a certain degree of corrosion resistance. A corrosion resistant cemented carbide generally has a binder phase consisting of Co, Ni, Cr and Mo where the Cr and/or Mo act as corrosion inhibiting additions. An example of such a cemented carbide with a medium WC grain size is disclosed in EP 28 620. EP 568 584 discloses the use of a corrosion resistant cemented carbide with submicron WC grain size with excellent properties particularly for tools used in the wood industry.
A critical component of subsea oil/gas production systems is the choke trim components, the primary function of which is to control the pressure and flow of well products. Under severe conditions of multi flow media, these components may suffer from extreme mass loss by exposure to solid particle erosion, acidic corrosion erosion-corrosion synergy and cavitation mechanisms even when fitted with cemented carbide trims.
The opportunity to maintain or replace such equipment in the field especially in offshore deep water sites is limited by weather conditions. It is therefore essential that reliable and predictable products form part of the subsea system.
The composition of the cemented carbide grades presently used for withstanding conditions of service in this type of environment generally consist of tungsten carbide (WC) as the hard component and cobalt (Co) or nickel (Ni) as the binder material to cement together the WC crystals.
To meet the demands of hardness and toughness, the amount of binder and/or the WC grain size are varied and cobalt is generally accepted as the optimum binder constituent. Where corrosion resistance is the predominant consideration then the binder material is usually of a nickel or a nickel+chromium (Ni+Cr) composition.
Analogous to stainless steels, Cr and Ni alloys have improved passivity by reducing the critical currents involved in corrosion, however (Cr+Ni) are not so resistant to halides (seawater) or inorganic acids. For these conditions the addition of molybdenum gives improved corrosion resistance in addition to improving binder strength of Ni.
Recent experimental work, including field trial evaluation, has proven that under high erosion conditions including a corrosion medium, the mechanism of mass loss is due not only to a combination of each individual corrosive condition, but the combination of corrosive conditions is synergistic.